OP I know how you feel, my last few years in high school have been horrible.
I've been expelled twice (I'm on a hat trick

) and find myself moving from school to school every year since 9 I even attended a TAFE in a rubbish suburb for one whole semester just because no school would accept me, if you want to know why I've been expelled it's because I was an extremely lazy student and never respected my teachers.
My lack of discipline even costed me the opportunity of a finishing a unit 3/4 subject in year 11.
Over the Summer holidays I've realised if I don't shape up for year 12 I'm going to be expelled and would probably end up in some sort of apprenticeship (no disrespect to anyone doing a trade) but I know that I want to get into uni.
It's not over OP, prove everyone wrong and here are a few things I recommend you do:
1)
USE ATAR NOTES TO YOUR ADVANTAGE ! Begin going on to the boards of the subjects your doing and read all the stickies, download all the notes and ask questions.
2) Invest some money into tutors, obviously your foundation isn't as good as the rest of average students so tutors are a good way to put you back on track.
3) Stay motivated. Always have a goal in your head and remind your self why your going to study hard in year 12 and leave all the rubbish from previous years in the past.
4)
Class of 2011: ATAR and SS ONLY Read this thread and go through every single page this motivated me so much imagine how happy these students are, and that could easily be you.
I don't have anymore off the top of my head, but if any are wrong feel free to correct me.
Good luck

Edit: I just remembered a story my brother told me of this girl who was in his class, in year 11 she was on the verge of expulsion, because she never studied and failed more than half her subjects.
Everyone lost faith in her, ever her own parents.
In year 12 she was out to prove everybody wrong, he told me she would always be studying even in that 5 minute break between 2 classes and waiting for the teacher she would be studying.
Long story short he told me she got the DUX of the school, and I promise you I'm not making up crap, she put in the hard yards and was rewarded.